The head involution defective (hid) locus is a previously uncharacterized gene located on the left arm of chromosome III of Drosophila melanogaster. At present, eleven recessive lethal mutations of this gene have been recovered and studied. These investigations reveal that the hid gene must be expressed sometime during the first 12 hours of embryogenesis so that the head of the fly larva will form correctly. In addition to its embryonic role, hid must also be expressed during the pupal stage to facilitate the construction of the adult fly. The role of the hid gene during embryogenesis appears to be the same as its role later in development: the wild-type hid product is required in specific groups of cells to permit these cells to be rearranged during normal development. The genetic control of cell movement and tissue reorganization, two fundamental developmental processes, has not been well studied in most organisms, including Drosophila. Further investigation of the hid gene may provide insight into a mechanism by which such processes are controlled; such a mechanism would then serve as a model for future studies. The hid DNA is being cloned, this will permit a more precise determination of when and where hid is expressed during development. In addition, the cloned DNA will be utilized to: (1) obtain monoclonal antibodies to the presumptive hid protein(s), and (2) identify other Drosophila genes that share homologous DNA sequences.